A brochure entitled "Diagnostic Monitoring Systems," University of Strathclyde, Scottish Power, and European Patent Application No. 488,719 describe a conventional measurement system which taps and analyzes interference signals using measurement probes.
To determine the relative phase angle of interference signals with respect to the useful voltage, the useful voltage is tapped in the phase in which an interference signal has occurred, and a signal proportional to the useful voltage is sent to the measurement system.
In investigating disturbances in the various phases, the appropriate phase must be selected for determination of the useful voltage signal to ascertain the phase angle of the interference signal.
Errors can easily occur in the correlation of phases with the interference signals, so that the wrong phase angle is assigned to an interference signal.
According to German Patent application No. 43 10 619, an interference signal in the high-frequency range is picked up using resonant circuits and conventional partial discharge measurement units, but this is not related to a useful signal (e.g., 50 Hz power supply voltage). Only high-frequency signals in different frequency ranges are detected.
U.S. Pat. No. 5,223,795 describes a method where a loss of high-frequency signal components relative to a fundamental frequency is detected. To monitor the high-frequency signal components of the voltage, the phase conductor is connected electrically or by way of a current transformer to a high-frequency analyzing unit as well as a device for analyzing the fundamental frequency.
German Patent No. 69,100,721 describes an arrangement for detecting conducted interference signals in conjunction with electric machines. Detection takes place exclusively through capacitive coupling of the measurement device to the system.
U.S. Pat. No. 5,396,180 describes a detection system for signals in a high-voltage switchgear where signals in the UHF range are obtained by operating a measurement electrode as an antenna, but information about the variation over time of the line frequency (see U.S. Pat. No. 5,396,180, FIG. 5, upper right) is evidently obtained from a separate device.